Background. Osteoarthritis (OA) of the knee is common and disabling. Exercise is effective in reducing pain and disability, but long-term adherence to exercise regimens is disappointing, and motivation to exercise in those with OA knee is poorly understood. Objectives. To examine the views of primary care patients with OA knee towards exercise, explore factors that determine the acceptability and motivation to exercise, and to identify barriers that limit its use. Method. Semi-structured interviews were conducted with 22 primary care patients with OA knee, six of whom also participated in a focus group for triangulation. Transcripts were coded into categories and themes to develop a conceptual framework and typology of exercise behaviour. Results. Exercise behaviour depended upon physical capacity to exercise; exercise beliefs and other factors such as enjoyment, social support, priority setting and context. Four types of patients were identified: ‘long-term sedentary’ who had never exercised; ‘long-term active’ who continued to exercise; ‘exercise retired’ who used to exercise, but had stopped because of their symptoms, and because they believed that exercise was damaging their joints; and ‘exercise converted’ who recently started to exercise, and preferred a gym because of the supervision and social support they received there. Conclusions. Several physical, cognitive and contextual factors, and a typology of exercise behaviour were identified that could be addressed in primary care consultations. The importance of gyms and GP referral schemes for people who are exercising for the first time, and the high level of patient satisfaction associated with these were highlighted.